(1) Field of the Invention
This invention relates to repairable display panels, and methods of repairing the panels, and more particularly to a display panel having redundant control elements and methods of repairing the panel wherein the defective elements are neutralized and isolated.
(2) Description of the Prior Art
Modern flat panel liquid crystal displays (LCD) have found wide usage, particularly in small portable computers capable of operating on batteries. Such displays, in general have a layer of liquid crystal material that is capable of changing its optical characteristics when exposed to an electric field. The LC material is usually sandwiched between two transparent or semitransparent glass substrates. One of the glass substrates has a matrix consisting of a set of spaced parallel conductive scan lines, and a second set of orthogonal spaced parallel conductive data lines. The areas defined by the lines define pixel areas. Each pixel contains a switching element, a transparent pixel electrode, and a storage capacitor. An array of switching elements, usually thin film transistors are associated with the scan and data lines, and operate to selectively apply electrical potentials to the pixel electrodes. The applied potential influences the liquid crystal material in the immediate area.
The potential is applied across a common conductive electrode on the opposed panel. FIG. 1 depicts a schematic diagram of a single pixel of a thin film transistor liquid crystal display panel. A field effect transistor 10, normally a very small thin film transistor (TFT) having an amorphous or polycrystalline silicon layer with a gate connected to a scan line 12, and the source joined to a data line 14. The drain is connected to pixel electrode 16. The area beneath pixel electrode 16 designates liquid crystal (LC) material. Numeral 17 indicates the common electrode on the opposed substrate panel above the pixel electrode 16. The area of pixel electrode 16 is equal to the area of the liquid crystal capacitor, and the dielectric of the liquid crystal capacitor is the liquid crystal itself. Numeral 18 designates a storage capacitor consisting part of pixel electrode 16, and a lower electrode plate 19 positioned below electrode plate 16, and separated by a dielectric. The gate insulating layer of the thin film transistor 10 can be used as the dielectric layer of the storage capacitor. In operation, the transistor 10 in response to signals on the scan and data lines, is selectively turned on to influence the liquid crystal material between plates 16 and 17 to produce an transmissible or opaque area which is dependent on the applied voltage of the pixel electrode.
Storage capacitors are usually used to improve the image quality of the TFT LCD. The storage capacitor 18 is usually electrically connected in parallel to the liquid crystal capacitor. It is necessary to improve the display performance for the following reasons: (1) it can reduce the cross talk between the pixel electrode 16 and the adjacent data line, (2) it can reduce the unacceptable voltage jump which occurs when the TFT is switched off, (3) it can increase the RC time constant that increases the gray level capability of the TFT LCD. However, the area of the storage capacitor is much larger than that of the transistor region, thus failure probability of the storage capacitor is much higher than that of the transistor. The panel is normally back lighted. An image is thus formed on the display panel, using many adjacent associated pixels, in response to signals applied to the scan and data lines.
During manufacture of liquid crystal display panels, defects in various lines, transistors and the storage capacitors, etc. can occur which will produce nonfunctioning individual pixels. The point defects will occur in the TFT LCD. The first is the TFT cannot charge the capacitors in the turn on period. We call this kind of failure the on-failure. The second is that the capacitors (which includes the liquid crystal capacitor and the storage capacitor) cannot keep the potential during the frame time. We call this kind of failure, the off-failure.
The frame time is defined as the period of a scan line signal. The on-failure is induced from the failure of TFT. The off-failure is induced from the leakage of TFT or the leakage of the storage capacitor. These nonfunctioning pixels can either be ignored, resulting in a poor quality panel or the defective panel discarded. Since display panels are relatively expensive, these are not desirable alternatives.
The U.S. Pat. No. 4,840,459 discloses a display panel with auxiliary repair lines that cross over the main scan and data lines of the display. The auxiliary lines can be electrically shorted to repair open circuit elements of the display panel. That is, this patent can repair the line defected to a point defect, however, it cannot be used to repair the point defect alone.